In recent years, a light-emitting device using a phosphor, which is excited with ultraviolet or visible light, is spreading rapidly. Specific light-emitting devices include a plasma display panel (PDP), a cold cathode discharge lamp used for a back light or the like of a liquid crystal display, a white light-emitting type LED lamp (white LED lamp) using a light-emitting diode (LED), and the like. When the conventional heavily used fluorescent lamps are added to the above, demands for the ultraviolet excitation type or visible light excitation type phosphor are increasing rapidly. With the increase of such demands and use, the quality that the phosphor is demanded to have, particularly a life property (phosphor luminance maintenance property when operating), has become higher in level because the light-emitting device has become common and highly sophisticated.
The ultraviolet excitation type phosphor is made to emit visible light (so-called light emission) by irradiation of ultraviolet rays having various wavelengths. As an ultraviolet ray supply source, it is general to use rare gas discharge (wavelength: mainly 147 nm and 172 nm), mercury (wavelength: mainly 254 nm), black light (wavelength: mainly 360 nm), ultraviolet LED (wavelength: mainly 380 nm), and the like, which are selectively used depending on an applying device. For example, the rare gas discharge is used for the plasma display panel, the mercury is used for the cold cathode discharge lamp for the back light and the fluorescent lamp, and the ultraviolet LED (ultraviolet ray-emitting LED) is used for the white LED lamp.
The phosphor is made to emit light by ultraviolet rays having various wavelengths, and its light-emitting property is peculiar to the phosphor. For example, a red light-emitting Y2O3:Eu phosphor which is used for the cold cathode discharge lamp and the fluorescent lamp is made to emit light efficiently by ultraviolet rays of 147 nm, 172 nm and 254 nm but not made to emit light substantially by ultraviolet rays of 360 nm and 380 nm. A blue light-emitting BaMgAl10O17:Eu phosphor used for the plasma display panel is made to emit light efficiently by all ultraviolet rays of 147 nm, 172 nm, 254 nm, 360 nm and 380 nm.
As phosphor materials for the light-emitting device described above, phosphors of various compositions have been developed. For example, ultraviolet excitation type phosphors used for a plasma display panel, a rare gas discharge lamp and the like are described in Patent Reference 1 and Patent Reference 2. The phosphors described in these Patent Literatures were improved their compositions to improve an emission intensity and a luminescent maintenance factor. Specifically, Patent Literature 1 describes an ultraviolet excitation type phosphor having a luminance maintenance factor of 97% after a continuous light-emitting operation of 1000 hours.
In recent years, the light-emitting device is being used extensively in various fields, such as the back light of the plasma display panel and the liquid crystal display, and its use methods are becoming available in great variety. Therefore, there was conventionally no problem if the luminance maintenance factor after a continuous operation of about 1000 hours was excellent. But, a higher luminance maintenance factor is demanded in these years. Specifically, an excellent luminance maintenance factor is demanded even after a lapse of 3000 hours or more, e.g., about 5000 hours. However, the conventional phosphor does not always provide a luminance maintenance factor with a good result after a continuous operation for more than 3000 hours.
[Patent Reference 1] JP-A 2002-348571 (KOKAI)
[Patent Reference 2] JP-A 2004-203980 (KOKAI)